1. Field of the Invention
The present invention relates to an X-ray computed tomographic apparatus including a rotating part which rotates in order to acquire an X-ray tomographic image of a subject.
2. Description of the Related Art
An X-ray computed tomographic apparatus (hereinafter referred to as “X-ray CT (Computed Tomography) apparatus”), which provides the information on a subject as an image on the basis of the intensity of X-rays transmitted through the subject, plays an important role in many medical acts, including diagnosis of diseases, medical care, surgery planning, etc.
Such an X-ray CT apparatus has an annular rotary base. This rotary base is supported so as to be rotatable about the axial center thereof, and various devices, such as an X-ray tube, an X-ray detector, and a high-voltage generator, are fixed inside the rotary base.
Rotation of the rotary base will change the orientation of the X-ray tube and the X-ray detector. In the X-ray CT apparatus, photographing of a subject from various directions is made using a change in the orientation of the above-mentioned X-ray tube and X-ray detector (for example, refer to JP-A No. 8-52130).
Meanwhile, since the X-ray tube, the X-ray detector, the high-voltage generator, etc. have heavy weight, when the rotary base rotates at high speed, a great centrifugal force may be generated in heavy loads, such as the X-ray tube, the X-ray detector, and the high-voltage generator, which are fixed inside the rotary base, and consequently the rotary base may be deformed.
Since the angle of the X-rays radiated from the X-ray tube will deviate if the installation angle of the X-ray tube gets wrong due to deformation of the rotary base, it becomes impossible for the X-ray detector to detect the X-rays correctly. Moreover, when deviation occurs in an X-ray path, the position where X-rays are detected will change and deterioration of an image will be caused.
Hereinafter, another case will be further described as an example with reference to FIG. 11.
FIG. 11 is a sectional view showing the configuration of a conventional rotating part 40. As shown in FIG. 11, a rotating part 40 which can rotate continuously at high speed (for example, greater than 1 second/rotation) in order to acquire an X-ray tomographic image of a subject is provided in a gantry of a conventional X-ray CT apparatus. This rotating part 40 is rotatably held in a frame 23 via a bearing 91. An X-ray tube unit 41, an X-ray detecting unit 42, a signal amplification unit 43, and other units (not shown) are attached to the rotating part 40. Hereinafter, each of these units may be called a rotating part unit. Each of these rotating part units is attached to a ring-shaped flat plate part 48 (flat plate) centering on a rotational axis N.
A drum-shaped drum part 49 is formed in an outer peripheral end of the flat plate part 48. Hereinbelow, a combination of the flat plate part 48 and the drum part 49 is referred to as a rotary base 52. The rotary base 52 is held in the frame 23 via the bearing 91. The bearing 91 is sandwiched by the flat plate part 48 and the bearing holding part 24.
An X-ray CT apparatus in which each rotating part unit is attached to the inside of the drum part 49 is disclosed (see, for example, JP-A No. 2000-116641). In such a rotating part 40, the center of gravity of each rotating part unit in the direction of the rotational axis N of the rotating part 40 does not often coincide with the position of the flat plate part 48. For example, the X-ray tube unit 41 and the X-ray detecting unit 42 are disposed such that the X-rays radiated from the X-ray tube unit 41 and transmitted through a subject is detected by the X-ray detecting unit 42 disposed opposite to the X-ray tube unit 41, with the subject therebetween. That, the X-ray tube unit 41 is disposed so as to form a transmission surface of X-rays parallel to the flat plate part 48 in a position away from the flat plate part in the direction of the rotational axis N. In this case, the center of gravity of the X-ray tube unit 41 is often in the vicinity of a position where X-rays are generated. Specifically, heavy things, such as a rotor (anode) and protective lead for shielding leakage X-rays are disposed in the vicinity of the X-ray generated position. Accordingly, the center of gravity of the X-ray tube unit 41 will be in a position that is away from the flat plate part 48 in the direction of the rotational axis N. Further, since the X-ray detecting unit 42 is disposed on the side of an X-ray transmission surface of the flat plate part 48, the center of gravity of the X-ray detecting unit 42 will be in a position that is away from the flat plate part 48 in the direction of the rotational axis N, like the center of gravity of the X-ray tube unit 41. Moreover, since other rotating part units are disposed on the same side as the X-ray tube unit 41 and X-ray detecting unit 42 with respect to the flat plate part 48 from the viewpoint of easiness of work at the time of maintenance, the center of gravity often deviates, similar to the X-ray tube unit 41 or X-ray detecting unit 42.
When the rotating part 40 rotates at high speed, a centrifugal force will act on the center of gravity of each rotating part unit. For example, a centrifugal force F which acts on the X-ray tube unit 41 is shown in FIG. 11 as an example. For example, in a case where the distance from the rotational axis N to the center of gravity of the X-ray tube unit 41 is 600 mm and rotational speed is 0.5 second/rotation, the gravitational acceleration of about 10 G is generated. Further, in a case where the distance from the rotational axis N to the center of gravity of the X-ray tube unit 41 is 600 mm and the rotational speed is 0.4 seconds/rotation, the gravitational acceleration of about 15 G is generated.
Meanwhile, if the center of gravity of the X-ray tube unit 41 has deviated from the position of the flat plate part 48 in the direction of the rotational axis N as mentioned above, a component force F1 of the centrifugal force F which acts on each rotating part unit when the rotating part 40 rotates at high speed may act on a holding position of the rotary base 52 as a bending moment, and the flat plate part 48 may be deformed accordingly. Similarly, the centrifugal force may also act on the X-ray detecting unit 42 or other rotating part units to deform the flat plate part 48. Although there is a difference in deformation volume even in a case where each rotating part unit is attached to the inside of the drum part 49 as disclosed in JP-A No. 2000-116641, deformation is caused in the flat plate part 48 similarly. The deformation of the flat plate part 48 based on the centrifugal force F makes it difficult to ensure the degree of precision of the position of the X-ray transmission surface at the time of high-speed rotation.
Thus, there is also a case where the X-ray transmission surface is made wide so that the X-ray detecting unit 42 can detect X-rays with a high degree of precision. In this case, however, the amount of exposure of a subject will be increased. Further, in a case where the degree of deformation of the flat plate part 48 is great, breakage of the rotary base 52, abnormality of a CT image due to shift of CT values, etc. may be caused.
On the other hand, although there is a demand for shortening the time required for photographing a moving object, for example, a heart, etc., that is, a demand for photographing the moving object while being made to rotate at higher speed so that the object can be observed as if it stands still, the above-mentioned deformation will be increased due to the higher-speed rotation.
Further, there is also a demand for photographing an object while the rotational speed is changed. Since a centrifugal force will change if rotational speed is changed, deformation volume will also change with the change in rotational speed. Therefore, there is a fear that abnormality of a CT image may be further caused due to the change in deformation volume.
Further, although it is considered that the strength of the flat plate part 48 is raised by increasing the thickness thereof, the rotary base 52 is often fabricated of aluminum cast. In this case, if the thickness of the rotary base is increased partially, a problem occurs in flow or cooling of molten metal of the cast. As a result, there is a fear that, for example, an internal stress may be generated or the degree of precision of a shape may deteriorate. On the other hand, if the whole rotary base including the drum part 49 is thickened, the weight thereof increases, and thereby a motor to drive the rotary base is also required to be enlarged. As a result, there is a problem that the X-ray CT apparatus may be enlarged.
Further, depending on a method of reinforcing the rotary base 52, lowering of the workability at the time of maintenance as described above may be caused.